1. Field of the Invention
This invention relates generally to oscillator circuits and, more particularly, to a tunable differential oscillator that optionally may have a voltage-controllable oscillation frequency.
2. Related Art
An oscillator is an electric or electronic circuit that produces an output signal that oscillates at a principal oscillation frequency and, in some instances, contains harmonic frequency signals (i.e., signals having frequencies that are multiples of the principal oscillation frequency). Some oscillators have a principal oscillation frequency that may be varied by the application of a suitable control voltage. Oscillators of this type, called voltage-controlled oscillators or VCO""s, are used in many communication and other signal-processing applications. Because suppression or reduction of higher-order harmonics of the principal oscillation frequency and other noise in the output signal of a VCO is often desirable, some prior-art VCO""s have been designed with so-called xe2x80x9cdifferentialxe2x80x9d outputs.
One application among many for which differential VCO""s and other oscillators have been used is in channel selection (i.e., tuning) for hand-held cellular telephones and other communication devices. Over time, efforts have been made to design such communication devices to be small, light-weight, and inexpensive to manufacture. However, prior oscillators used in such devices have employed numerous electrical and/or electronic components. In some cases, the prior oscillators have required a differential control voltage to effectuate tuning of the principal oscillation frequency of the VCO output signal. Consequently, prior differential VCO""s and other oscillators are undesirably large and expensive. This hinders the efforts to reduce the size, weight, and cost of hand-held communication devices employing such oscillators.
This invention provides oscillators that employ relatively few components thus reducing their complexity, size, weight, and cost. Optionally, differential oscillators may be tunable by, for example, the application of a single tuning signal to a voltage-controlled capacitor or varactor.
In one embodiment, an oscillator includes first and second switching elements, each having a respective control terminal and respective first and second conduction terminals. The control terminal of the second switching element is coupled to the first conduction terminal of the first switching element, and the control terminal of the first switching element is coupled to the first conduction terminal of the second switching element.
In another embodiment, the control terminal of each switching element is directly coupled to the first conduction terminal of the other switching element. For example, the first and second switching elements may comprise transistors. Thus, the first and second switching elements may comprise bipolar transistors, and the control terminals and first and second conduction terminals may comprise the bases, collectors, and emitters of the bipolar transistors.
In addition, some embodiments of oscillators of the invention may include a first capacitive element coupled between the control terminal of the first switching element and a first reference node, and a second capacitive element coupled between the control terminal of the second switching element and a second reference node. The first and second reference nodes may be the same or different, and the first and second capacitive elements may, but need not have substantially equal capacitance. In some embodiments, one of the first and second capacitive elements may have a variable capacitance and may comprise a varactor.
Other embodiments of oscillators according to the invention also may include a first inductive element coupled between the control terminal of the first switching element and a first reference node, and a second inductive element coupled between the control terminal of the second switching element and a second reference node. In some embodiments, the first and second reference nodes may be commonly connected, and a voltage supply may be coupled to the commonly connected first and second reference nodes. Further, the first and second inductive elements may have substantially equal inductance.
Also, the second conduction terminals of the first and second switching elements may be connected to one another in some embodiments, and a resistive element may be coupled between the commonly connected second conduction terminals of the switching elements and a reference node.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.